1Dr.S. Nayak Digestion Digestion The human diet contains carbohydrates, fat and proteins, which are high molecular weight complex compounds. All have to

11Dr.S. NayakDr.S. Nayak

Digestion Digestion

The human diet contains carbohydrates, fat and The human diet contains carbohydrates, fat and proteins, which are high molecular weight complex proteins, which are high molecular weight complex compounds. compounds. All have to be digested before get absorbedAll have to be digested before get absorbed

Digestion of carbohydratesDigestion of carbohydrates The major carbohydrates of our diet are The major carbohydrates of our diet are starch starch and and glycogenglycogen The polysaccharides are hydrolyzed to simple sugars The polysaccharides are hydrolyzed to simple sugars through the action of number of enzymes. through the action of number of enzymes. Salivary amylase hydrolyses Salivary amylase hydrolyses -1,4-glycosidic -1,4-glycosidic linkages of linkages of polysaccharide chain and produce disaccharides and polysaccharide chain and produce disaccharides and monosaccharides. monosaccharides. The further digestion takes place in the small intestine The further digestion takes place in the small intestine by theby the intestinal enzymes, which hydrolyze terminal intestinal enzymes, which hydrolyze terminal -1,4--1,4-glycosidic glycosidic linkage. linkage.


Entry of acidic contents of stomach into duodenum, Entry of acidic contents of stomach into duodenum, stimulate the mucosal cells to release stimulate the mucosal cells to release secretin secretin and and cholecystokinin cholecystokinin

SecretinSecretin stimulates the pancreas to release stimulates the pancreas to release bicarbonate and waterbicarbonate and water to neutralize the acidic to neutralize the acidic chyme from the stomach chyme from the stomach

CholecystokininCholecystokinin stimulates the production of stimulates the production of digestive enzymes including digestive enzymes including pancreatic amylaspancreatic amylase e

Pancreatic amylase digest the polysaccharides to Pancreatic amylase digest the polysaccharides to maltose, isomaltose and a limit dextrin.maltose, isomaltose and a limit dextrin.

Disaccharidases such as Disaccharidases such as maltasemaltase, , lactaselactase and and sucrasesucrase digest disaccharides like maltose, lactose and sucrose digest disaccharides like maltose, lactose and sucrose respectively into their respective monosaccharide respectively into their respective monosaccharide units. units.




Cellulose is not digested further because Cellulose is not digested further because humans does not produce humans does not produce β 1,4-β 1,4-endoglucosidase endoglucosidase in digestive juice. in digestive juice.

Cellulose helps in easy peristalsis and provides Cellulose helps in easy peristalsis and provides bulk to the feces.bulk to the feces.

Lactase deficiency leads toLactase deficiency leads to lactose lactose intoleranceintolerance..

Absorption of monosaccharides Absorption of monosaccharides The The galactose galactose and and glucoseglucose are absorbed very are absorbed very

rapidly by the rapidly by the active processactive process, , Fructose and mannoseFructose and mannose are absorbed by a Na are absorbed by a Na++

independent facilitative transport mechanismindependent facilitative transport mechanism


Digestion of proteins Digestion of proteins

In the stomachIn the stomach: : The protein does not undergo any digestion The protein does not undergo any digestion

in the mouth. in the mouth. When it enters the stomach, it stimulates the When it enters the stomach, it stimulates the

secretion of the hormone secretion of the hormone gastringastrin, from gastric , from gastric mucosal cells. mucosal cells.

The gastrin stimulates the release of gastric The gastrin stimulates the release of gastric juice containing HCl and pepsinogen [rennin juice containing HCl and pepsinogen [rennin in infants]. in infants].

The HCl is secreted by parietal cells, which The HCl is secreted by parietal cells, which unfolds the proteins and activates the unfolds the proteins and activates the proteolytic enzyme pepsin. proteolytic enzyme pepsin.


Pepsin secreted by chief cells as pepsinogen Pepsin secreted by chief cells as pepsinogen & converted to pepsin by HCl. This activation & converted to pepsin by HCl. This activation is called as zymogen activation is called as zymogen activation

Pepsin converts Pepsin converts protein polypeptidesprotein polypeptides into into tripeptides, dipeptides and amino acids. tripeptides, dipeptides and amino acids.

Pepsin specifically hydrolyses the peptide Pepsin specifically hydrolyses the peptide bonds of protein involving the aromatic amino bonds of protein involving the aromatic amino acids (Phenylalanine, tyrosine or tryptophan) acids (Phenylalanine, tyrosine or tryptophan) or acidic amino acids (aspartic acid and or acidic amino acids (aspartic acid and glutamic acid)glutamic acid)

Rennin Rennin in infants is also called as chymosin or in infants is also called as chymosin or rennet. rennet.

Digestion in the intestineDigestion in the intestine: : When the acidic contents from the stomach When the acidic contents from the stomach

pass into the intestine, the low pH triggers the pass into the intestine, the low pH triggers the secretion of the hormones such as secretion of the hormones such as cholecystokinincholecystokinin and and secretin.secretin.


Secretin stimulates the release of bicarbonate Secretin stimulates the release of bicarbonate and pancreatic juice from pancreas into the and pancreatic juice from pancreas into the small intestinesmall intestine

Cholecystokinin stimulates the secretion of Cholecystokinin stimulates the secretion of pancreatic endo and exopeptidases pancreatic endo and exopeptidases

Endopeptidases cleave internal peptide bonds Endopeptidases cleave internal peptide bonds of proteins to convert into smaller peptidesof proteins to convert into smaller peptides

The endopeptidases: trypsin, chymotrypsin, The endopeptidases: trypsin, chymotrypsin,

elastase, which are secreted in pro-enzyme elastase, which are secreted in pro-enzyme forms and are converted to active forms by forms and are converted to active forms by enteropeptidase (enterokinase) enteropeptidase (enterokinase)


Trypsin Trypsin hydrolyses peptide bonds whose carboxyl hydrolyses peptide bonds whose carboxyl groups are contributed by groups are contributed by lysine lysine andand arginine arginine

Chymotrypsin specifically hydrolyses the peptide Chymotrypsin specifically hydrolyses the peptide bonds involving the carboxyl group aromatic amino bonds involving the carboxyl group aromatic amino acids (Phenylalanine, tyrosine or tryptophan)acids (Phenylalanine, tyrosine or tryptophan)It also splits peptide bonds of leucine, methionine It also splits peptide bonds of leucine, methionine asparagine and histidineasparagine and histidineElastase hydrolyses those peptide bonds formed by Elastase hydrolyses those peptide bonds formed by non-polar amino acids, such as alanine, serine and non-polar amino acids, such as alanine, serine and glycineglycine

Exopeptidases: carboxypeptidase and aminopeptidaseExopeptidases: carboxypeptidase and aminopeptidase Carboxypeptidase is a composition of pancreatic Carboxypeptidase is a composition of pancreatic

juice and hydrolyses the first peptide bond from the juice and hydrolyses the first peptide bond from the free carboxyl endfree carboxyl end

Aminopeptidase hydrolyses the first peptide bond Aminopeptidase hydrolyses the first peptide bond from the free amino terminal endfrom the free amino terminal end


DIGESTION AND ABSORPTION OF LIPIDSDIGESTION AND ABSORPTION OF LIPIDS

Digestion of lipids depends on the bile salts Digestion of lipids depends on the bile salts for the emulsification. for the emulsification.

The heat of the stomach is important in The heat of the stomach is important in liquefying dietary lipids. liquefying dietary lipids.

Lipids inhibit gastric motility and so retard the Lipids inhibit gastric motility and so retard the evacuation of the stomach evacuation of the stomach

The gastric lipase and lingual lipase of chyme The gastric lipase and lingual lipase of chyme are active only at neutral pH are active only at neutral pH

In adults no digestion takes place in stomach In adults no digestion takes place in stomach due to acidic pH due to acidic pH

The hydrophilic short and medium chain fatty The hydrophilic short and medium chain fatty acids are absorbed via stomach wall and enter acids are absorbed via stomach wall and enter the portal vein. the portal vein.

The longer chain fatty acids dissolve in the The longer chain fatty acids dissolve in the diet and pass into the duodenum diet and pass into the duodenum


Entry of acidic chyme from the stomach into Entry of acidic chyme from the stomach into the duodenum stimulates the secretion of the duodenum stimulates the secretion of enteric hormones like enteric hormones like secretinsecretin and and cholecystokinincholecystokinin by the mucosal cells of by the mucosal cells of duodenumduodenum

This cholecystokinin acts on the gallbladder, This cholecystokinin acts on the gallbladder,

causing it to contract to release bile salts into causing it to contract to release bile salts into the small intestinethe small intestine

Cholecystokinin also acts on the exocrine Cholecystokinin also acts on the exocrine cells of the pancreas, causing them to release cells of the pancreas, causing them to release digestive enzymes including lipasedigestive enzymes including lipase

The same cholecystokinin also decreases The same cholecystokinin also decreases gastric motility, which results in a slow gastric motility, which results in a slow release of the gastric contents into small release of the gastric contents into small intestine.intestine.


The secretin causes the pancreas to release The secretin causes the pancreas to release a bicarbonate rich solution which neutralizes a bicarbonate rich solution which neutralizes the acidic chyme and changes the pH to the the acidic chyme and changes the pH to the alkaline sidealkaline side

The formation of alkaline pH of the content The formation of alkaline pH of the content is very important for the action of lipase and is very important for the action of lipase and intestinal enzymesintestinal enzymes

The bile enters the duodenum and provides The bile enters the duodenum and provides

the emulsifying action. After emulsification, the emulsifying action. After emulsification, the lipolytic enzymes such as lipase, the lipolytic enzymes such as lipase, phospholipase A2 and cholesterol esterase phospholipase A2 and cholesterol esterase present in the pancreatic juice hydrolyse present in the pancreatic juice hydrolyse lipids. lipids.


Dietary glycerophospholipds are digested by Dietary glycerophospholipds are digested by pancreatic phospholipase-A2 . This enzyme pancreatic phospholipase-A2 . This enzyme catalyzing the hydrolysis of fatty acid residues at catalyzing the hydrolysis of fatty acid residues at the 2the 2ndnd position of the glycerophospholipid, leaving position of the glycerophospholipid, leaving lysophospholipidslysophospholipids

This lysophospholipid enter the mucosal cell or This lysophospholipid enter the mucosal cell or degraded further by lysophospholipase enzyme degraded further by lysophospholipase enzyme (secreted by intestinal cells) to remove final fatty (secreted by intestinal cells) to remove final fatty acid residueacid residue

Inside the Inside the mucosal cellsmucosal cells fats are re-synthesized fats are re-synthesized and converted to chylomicron and transported to and converted to chylomicron and transported to blood via lymphatic vessel blood via lymphatic vessel

Fatty acids less than 10 carbon atoms along with Fatty acids less than 10 carbon atoms along with glycerol are carried by portal blood to the liver. glycerol are carried by portal blood to the liver.


The long chain free fatty acids, free cholesterol, 2-The long chain free fatty acids, free cholesterol, 2-monoglyceride and 1-monoglyceride and monoglyceride and 1-monoglyceride and lysophospholipid together with bile salts form lysophospholipid together with bile salts form mixed mixed micellesmicelles. .

The bile salts aggregate with their hydrophobic region The bile salts aggregate with their hydrophobic region placed internally and hydrophilic region facing the placed internally and hydrophilic region facing the water medium and makes the micelle water soluble. water medium and makes the micelle water soluble. The glycerides and long chain fatty acids in these The glycerides and long chain fatty acids in these micelles are transported into the intestinal mucosal micelles are transported into the intestinal mucosal cells leaving bile salts in the medium itself. The bile cells leaving bile salts in the medium itself. The bile salts are reabsorbed in the intestine and returned to salts are reabsorbed in the intestine and returned to the liver by the portal vein for re-secretion into the the liver by the portal vein for re-secretion into the bile. This process is called bile. This process is called enterohepatic circulationenterohepatic circulation of of bile saltsbile salts

The short and medium chain fatty acids are absorbed The short and medium chain fatty acids are absorbed

directly into the intestinal epithelial cells and enters directly into the intestinal epithelial cells and enters the portal blood to reach the liverthe portal blood to reach the liver



The 1-monoacylglycerol are further The 1-monoacylglycerol are further hydrolysed in the intestinal mucosal cell by hydrolysed in the intestinal mucosal cell by intestinal lipase intestinal lipase

The 2-monoacylglycerol are reconverted to The 2-monoacylglycerol are reconverted to triglyceride. The utilization of fatty acids triglyceride. The utilization of fatty acids inside the mucosal cell for the re-synthesis inside the mucosal cell for the re-synthesis of triacylglycerol needs the activation to of triacylglycerol needs the activation to acyl-CoA by thiokinase enzyme acyl-CoA by thiokinase enzyme

The absorbed lysophopholipids and The absorbed lysophopholipids and cholesterol are also recycled with acyl-CoA cholesterol are also recycled with acyl-CoA to regenerate phopholipids and cholesterol to regenerate phopholipids and cholesterol esters. esters.


The triacylglycerol, phospholipid, cholesterol The triacylglycerol, phospholipid, cholesterol ester synthesized in the intestinal mucosal cell ester synthesized in the intestinal mucosal cell and the absorbed fat soluble vitamins are and the absorbed fat soluble vitamins are transported from the mucosal cells into the transported from the mucosal cells into the lymph in the form of chylomicronlymph in the form of chylomicron

The absorbed lipids are either oxidized mainly The absorbed lipids are either oxidized mainly in the liver or stored in the depots (adipose in the liver or stored in the depots (adipose tissue)tissue)

For utilization by the body, triglycerides are For utilization by the body, triglycerides are first hydrolysed by lipase to release glycerol first hydrolysed by lipase to release glycerol and free fatty acids and free fatty acids

Glycerol is converted to glucose by Glycerol is converted to glucose by gluconeogenesis or enters into glycolysis. gluconeogenesis or enters into glycolysis. Fatty acids are oxidized to COFatty acids are oxidized to CO22 and H and H22O with O with the liberation of large amount of energy. the liberation of large amount of energy.



Vitamins and MineralsVitamins and Minerals DUODENUM: Fe, Ca, Mg, ZnDUODENUM: Fe, Ca, Mg, Zn JEJUNUM: Vit C, B1 B2, B6, Folic AcidJEJUNUM: Vit C, B1 B2, B6, Folic Acid ILEUM: Fat soluble vitamins are absorbed via micelles; ILEUM: Fat soluble vitamins are absorbed via micelles;

B12 B12 COLON: Na; K; vit K from bacterial action COLON: Na; K; vit K from bacterial action

ABSORPTION OF MINERALSABSORPTION OF MINERALSPHASE 1: INTRALUMINALPHASE 1: INTRALUMINAL Chemical reactions and interactions in the Chemical reactions and interactions in the

stomach and intestinesstomach and intestines Cations (eg. Ca Cations (eg. Ca 2+2+): influenced by pH of luminal ): influenced by pH of luminal

contents and composition of chyme from contents and composition of chyme from stomachstomach

Soluble in acid pH of stomach but form insoluble Soluble in acid pH of stomach but form insoluble hydroxides in the higher pH of the small hydroxides in the higher pH of the small intestineintestine

Kept available for absorption by ligands such as Kept available for absorption by ligands such as amino acids, organic acids, sugarsamino acids, organic acids, sugars


PHASE 2: TRANSLOCATIONPHASE 2: TRANSLOCATION Passage across the membrane into the intestinal Passage across the membrane into the intestinal

mucosal cellmucosal cell Small anions: via simple diffusionSmall anions: via simple diffusion Cations: facilitated diffusion or active transport . Often Cations: facilitated diffusion or active transport . Often

more than one mechanism is available depending on more than one mechanism is available depending on concentration of trace elementconcentration of trace element

PHASE 3: MOBILIZATIONPHASE 3: MOBILIZATION Transport across the serosal surfaces of cell into blood Transport across the serosal surfaces of cell into blood

or Sequestered within the cell or BOTH or Sequestered within the cell or BOTH Fe and Zn are either bound to proteins within the cell Fe and Zn are either bound to proteins within the cell

or added to the intracellular pool.or added to the intracellular pool. Bound forms can be added to the pool or remain Bound forms can be added to the pool or remain

bound and are lost via desquamationbound and are lost via desquamation

Reference: Essentials of Biochemistry byReference: Essentials of Biochemistry byDr S NayakDr S Nayak

Documents

1Dr.S. Nayak Digestion Digestion The human diet contains carbohydrates, fat and proteins, which are high molecular weight complex compounds. All have to